Solution

Exactly 6 lines of code needed to be modified to complete the assignment. First, the Expr type needed to be modified to represent sums and products of arbitrary numbers of expressions.

This:

data Expr = Num Int
          | Neg Expr
          | Add Expr Expr
          | Mul Expr Expr

becomes this:

data Expr = Num Int
          | Neg Expr
          | Add [Expr]
          | Mul [Expr]

Second, the eval function needs to be modified to evaluate Add and Mul.

This:

eval (Add x y) = eval x + eval y
eval (Mul x y) = eval x * eval y

becomes this:

eval (Add xs) = sum $ map eval xs
eval (Mul xs) = product $ map eval xs

Finally, the parsers for Add and Mul need to be modified to parse multiple expressions and return a list to the constructors (Add and Mul are now no longer constructors that take two Expr values but that take a single value of type [Expr]).

This:

add :: Parser Expr
add = Add <$> ((spaceChar '+') *> expr) <*> expr

becomes this:

add :: Parser Expr
add = Add <$> ((spaceChar '+') *> some expr)

and similarly for mul.